Adult-onset inborn errors of metabolism (IEM) are very rare and their details remain unknown. Diagnosis, age at onset, clinical findings, and outcome of patients with IEM over 20 years old whose diagnosis were made at Shimane University between 2001 and 2010 were investigated. Out of 386 IEM cases identified, 24 cases (6.4%) were diagnosed during adulthood, among which 15 patients were adult onset. There were 11 cases with alkaptonuria, 6 patients with organic acidemia without alkaptonuria, 4 cases with urea cycle disorders and 3 subjects with fatty acid oxidation disorders. Outcome of adult-onset IEM are better than those of child-onset; however, some patients suffered from progressive neurological disorders because of the time lag before the diagnosis are determined after the onset. Blood acylcarnitines and urine organic acids should be analyzed for adult patients with unknown regression or mental retardation for early diagnosis.
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